1. Field of the Invention
The present disclosure relates generally to a magnetic particle carrying device such as a developing agent carrier, a developing unit, a process cartridge, an image forming apparatus using the magnetic particle carrying device, and a surface treatment method for the magnetic particle carrying device.
2. Description of the Background Art
Typically, an image forming apparatus includes a photosensitive drum and a magnetic particle carrying device (e.g., a developing agent carrier) having a hollow structure (e.g., a developing sleeve). In such image forming apparatus, developing agent is carried on an external surface of the developing sleeve and then transported to the photosensitive drum for an image forming operation.
Such developing sleeve has an external surface subjected to a surface roughening process, for example sandblasting the external surface or forming grooves therein, so that the developing agent can be reliably carried on the developing sleeve.
Further, the developing sleeve has an external surface randomly formed with a number of depressions, each having a substantially elliptical shape when viewed from above. Such depressions in the developing sleeve are of two types, each defined by an orientation of a long axis of the elliptical depression. In a first type of depression, the long axis of the elliptical depression is substantially aligned with an axis of the developing sleeve, whereas in a second type of depression, the long axis of the depression is substantially aligned with a circumferential direction of the developing sleeve, that is, a direction perpendicular to the axial direction. The number of depressions of each type is typically unequal, with the first type predominant.
When a developing sleeve, having a hollow structure, is treated by the above-described sandblasting process to form concavities and convexities on its external surface, such concavities and convexities are relatively small. Accordingly, under repeated printing operations, such concavities and convexities are gradually scraped flat or nearly flat by the developing agent or the like, gradually reducing the amount of developing agent that the developing sleeve can transport and adversely affecting image quality, resulting, for example, in faint images.
The amount of developing agent that the developing sleeve can transport is enhanced by forming larger concavities and convexities on a surface of the developing sleeve, again by sandblasting. However, such an approach has drawbacks. For example, the more powerful sandblasting that is required to form larger concavities and convexities can deform the developing sleeve itself, adversely affecting its rotation. Failure of the developing sleeve to rotate precisely can cause a predetermined gap set between the developing sleeve and the photosensitive drum to fluctuate, which may result in an unstable supply of the developing agent to the photosensitive drum and a consequent lack of appropriate toner concentration in the formed image.
Alternatively, as described above, grooves can be formed in the external surface of the developing sleeve. Such grooves can be larger than the concavities and convexities formed by the above-described sandblasting process, and larger also than the particles of magnetic carrier or the like contained in a developing agent. This larger size of the grooves prevents them from being as thoroughly or as rapidly abraded by the developing agent as the concavities formed by sandblasting tend to be, and therefore the amount of developing agent that can be transported by the developing sleeve does not deteriorate as greatly over time.
However, such developing sleeve may have an uneven distribution of developing agent across its external surface because the grooves can carry and transport greater amounts of developing agent than areas having no grooves, which may lead to uneven toner concentration in the resultantly produced images.
With respect to the above-described elliptical depressions formed in the external surface of the developing sleeve, these are larger or deeper than dents formed by conventional sandblasting. Therefore, the developing agent is less likely to abrade such elliptical depressions, and therefore the amount of developing agent that the developing sleeve can carry does not deteriorate over time and images having appropriate concentrations of toner can continue to be produced.
Further, because such depressions can be formed on the external surface of the developing sleeve randomly, the developing agent can be carried on the developing sleeve randomly as a whole, which means that the developing agent can be uniformly attracted to the developing sleeve as a whole. Therefore, such developing sleeve may suppress image concentration unevenness of resultantly produced images.
Further, as noted above, the depressions on the external surface of the developing sleeve include first type depressions, extending in the axial direction of the developing sleeve, and the second type depressions, extending in the circumferential direction of the developing sleeve, and the number of the first type depressions is greater than the number of the second type depressions on the external surface. Accordingly, the developing agent can be picked-up onto the developing sleeve along the axial direction of the developing sleeve. Therefore, even if the developing sleeve rotates, the picked-up developing agent is less likely to drop from the external surface of the developing sleeve. Accordingly, elliptical depressions may be able to carry as much developing agent as the above-described grooves do.
However, such depressions in the developing sleeve may include a relatively smaller number of depressions aligned in the circumferential direction of the developing sleeve. Accordingly, adhering density (or amount) of developing agent in the circumferential direction of the developing sleeve may become lower or uneven, and thereby image concentration unevenness in a sheet transport direction may not be effectively suppressed or prevented. In general, image concentration unevenness in a sheet transport direction is more recognizable compared to image concentration unevenness in a sheet width direction, which is perpendicular to the sheet transport direction.
In view of such background, a method or an apparatus capable of suppressing image concentration unevenness in a sheet transport direction is desired.